You said it happened when off roading in low range

This will heat up your trans to the max, especially if the ambient temp is high, no airflow over trans cooler at low speed and high load

Definitely add a trans temp gauge, like I did (after cooking one), I have decided to stop a couple of times when my trans temp exceeded 230F

Make sure your lock up solenoid is working, if not, that adds huge heat load

Dexron VI will handle the heat better...

Rob, how old is your radiator?
Do you have a trans cooler installed?

Radiator is around 18 years old, prior owner replaced when he got new factory rebuilt motor. he had good records of regular fluid maintenance. when I got it fluid was nice and clean, I have flushed it out with hose kit that splices in heater hose, it all came out clean then. Prestone 5 year fluid, 50 50 mix with distilled water.
I have a cooler for atf, plate type, about foot long, and about 7 inches wide or so. . I put in new thermostat a few years ago, and a FlowCooler water pump. belt is good, and crank pulley has been paint marked to prove that the balancer rubber is not causing slip. stock electric fan works, but atf cooler is in front of engine driven fan. fan clutch is from grand cherokee, and clutch does lock up as per test.. when hot can only hand spin fan about one or two blades worth of rotation.
engine can get several needle widths past the center mark, when wheeling hard, near to third of four marks on stock gage.
I try to always wheel in locked condition, have the 1/ 2 gear override switch.
transfer case is very low range 4.3 ratio, rear ends are 4.11, tires 33x12.5 MT

when slipped it was a hot August day in Hollister Hills Calif. Not scorching hot, but still hot.

got a laser gun for checking temps?

I hope an infrared gun...

Yeah. Infrared. Not like the ones used in Maui.

yes. but she hasnt slipped again.

Okay. But maybe you can isolate a hot spot or cold spot anyway?
Point it at the cooler lines at the trans and radiator?

Or, just hope for the best I guess.

Thats how I drive it, every single day🤞😆